Any discussion of the background art throughout the specification should in no way be considered as an admission that such art is widely known or forms part of common general knowledge in the field.
RCD protection technology was invented around the 1960's to provide a tertiary level of electrical protection against electrocution. RCD technology reacts to leakage of current from an electrical circuit to implement a protective function should that leakage current exceeds a level that is deemed to be likely to cause harm to humans.
Considerable work has been conducted over the decades as to what level of leakage current is reasonable to ensure adequate safety. Generally at power frequencies a maximum of 30 mA is considered the accepted limit. In certain applications this can reduce to 5 mA or increase to as much as 100 mA.
Over the last two decades or so electrical circuit current leakage or RCD technologies (referred within as current leakage) have progressively become widespread in domestic and industrial applications especially in the developed world. This is due to their inclusion in many national safety standards. However at the same time many deficiencies in the technology have become apparent. In particular it is being increasingly recognised that there are many circumstances where an RCD will not provide adequate protection, or where it will be susceptible to false triggers.
In the decade leading up to 2010 there were a number of technologies developed in an attempt to address one or more of these deficiencies. One earlier proposed technology sensed the voltage of the protective metal work of an appliance (irrespective of whether that metal work was connected effectively to local or remote ground). The technology, which will be referred to as voltage sensing technology, instituted a protective function if the voltage of the protective metal work moved away from supply neutral in a way that could potentially expose a human to harm through an electric shock. This technology provides protection against faults in systems where ground return paths are insufficient (too high resistance) to allow sufficient leakage current to trip the RCD technology. However, this technology is not readily applicable to MEN systems in light of the method of operation and the difficulties in achieving compliance with the required standards for protection circuits.
Further known systems monitor current flowing from protective metal work as an alternative to (or in combination with) monitoring the voltage of the protective metal work. Such systems found in, for example, PCT publication WO/2010/069011, relates to the other techniques discussed in that: (1) such sentinel (or fault) current is leakage from the electrical circuit and therefore related to RCD technology—although it is monitored in a different circuit location; and (2) such leakage currents are as a result of the voltage potential applied to the protective metal work by an internal fault to the Active conductor and such would also be seen by voltage sensing technologies.
As with voltage sensing, current flow fault sensing relies on an electrical reference (for current flow it is a current sink and for voltage sensing it is a reference voltage level). This is theoretically different from RCD circuit current leakage technology that monitors only current leakage (in the form of active/neutral imbalance) without reference to where that current is flowing (that is, the sink for such current).
Although the above technologies have a similar protection aim of identifying faults which might cause safety hazards, they use different fault-information collection strategies based respectively on current and voltage sensing at different locations within the circuit to achieve that aim. Importantly, the strategic differences are related to voltage reference, current sink, physical location, and fault-impedance differences.
As a result there is a correlation between the fault information gathered by each technique. For example, voltage sensing and current sensing technologies differ in that current sensing is highly dependent of the impedance of the fault. A high (non-critical) fault impedance may still result in a significant voltage on the chassis. However, such a voltage may not be capable of delivering sufficient current to cause harm. With regards to leakage current technologies, they may sense no imbalance in the circuit due to fault or circuit characteristics (that is, where earthing is poor), even when a significant fault occurs. In such cases voltage sensing or current sensing technologies may clearly see a fault signal.
An important difference between these forms of protection is demonstrated on consideration of floating earth installations. Current Leakage (RCD) technology is often used for well-earthed Multiple Earth Neutral (MEN) systems (also called TN) as it responds relatively quickly to any current leakage from the circuit due to a fault to earth (the most common type of fault). However, current leakage technology requires positive and significant fault current flow before it generates any fault indication. In poorly earthed environments (such as within TT and IT systems) such current flow may not occur and therefore RCD sensing will not receive any fault information.
Similarly frame and metal work voltage and current sensing technologies provide fault sensing in floating systems by providing an independent reference to either measure the result of a fault as active voltage on the metal chassis or facilitate a small current flow to generate a fault signal. Consequently, in a well-maintained MEN (TN) system, chassis or electrical metal work sensing may not provide a reliable fault signal.
In summary, known systems and devices may be adequate for some applications. For example, RCD technologies being most applicable to well earthed MEN or TN type installations while chassis and metal work voltage and current sensing technologies are most applicable to poor or floating earth installations (TT and IT).
In a further and more recent development, use has been made of both current and voltage sensing in a single protection device. A device of this type is the subject matter of Australian provisional patent application No. 2012903629, filed on 22 Aug. 2012. This device has a capability that spans a wider range of power system configurations and levels of degradation, from earthed-neutral (TN) to floating power configurations (IT), from new installations to aging and poorly maintained installations. Each technology is anticipated as independent and separable, with each independently monitoring their respective fault signals and when either threshold is reached activating a protective function.
Notwithstanding better performance than either of the earlier technologies many practical problems remain in gaining reliable and widespread use of such devices. These problems include:                A susceptibility to fault triggering as a result of electrical disturbances such as lighting and power line transients. These disturbances typically create earth currents through surge devices or circuit capacitances, or elevated Ground/Neutral voltages.        A susceptibility to noise-based false triggering;        A susceptibility to false or desensitized triggering as a result of electromagnetic coupling with adjacent circuits and other circuit effects.        A susceptibility to false or desensitized triggering caused by third party equipment failures upsetting common earthing and bonding networks.        A susceptibility to fault or desensitized triggering as a result of complicated earthing/bonding arrangements confusing, or cloaking the fault signal;        High relative cost of alternative techniques.        